Principal Components Granulometric Analysis of Tidally Dominated Depositional Environments

MITCHELL, STEVEN W., California State University, Bakersfield, CA, WILLIAM T. LONG, Occidental Petroleum Corporation, Houston, TX, and NANCY E. FRIEDRICH, Mount Holyoke College, South Hadley, MA

Sediments often are investigated by using mechanical sieve analysis (at 1/4 or 1/2 f intervals) to identify differences in weight-percent distributions between related samples, and thereby, to deduce variations in sediment sources and depositional processes. Similar granulometric data from groups of surface samples from two siliciclastic estuaries and one carbonate tidal creek have been clustered using principal components analysis. Subtle geographic trends in tidally dominated depositional processes and in sediment sources can be inferred from the clusters.

In Barnstable Harbor, Cape Cod, Massachusetts, the estuary can be subdivided into five major subenvironments, with tidal current intensities/directions and sediment sources (longshore transport or sediments weathering from the Sandwich Moraine) as controls.

In Morro Bay, San Luis Obispo County, California, all major environments (beach, dune, bay, delta, and fluvial) can be easily

distinguished; a wide variety of subenvironments can be recognized. In the delta, for example, distributary and tidal channels, topset, foreset, and bottomset zones, and delta front and distributary mouth bars produce different clusters based on differences in transport processes and grain sources.

On Pigeon Creek, San Salvador Island, Bahamas, twelve subenvironments can be recognized. Biogenic (Halimeda, Peneroplis, mixed skeletal), chemogenic (pelopids, aggregates), and detrital (lithoclastis of eroding Pleistocene limestone) are grain types which dominate. When combined with tidal current intensities/directions, grain sources produce subenvironments distributed parallel to tidal channels.

The investigation of the three modern environments indicates that principal components granulometric analysis is potentially a useful tool in recognizing subtle changes in transport processes and sediment sources preserved in ancient depositional sequences.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)